1. Field of the Invention
This invention relates to a method of recording and reproducing data on and from a rewritable optical disk on a sector-by-sector basis.
2. Description of the Prior Art
To meet requirements of high-density recording on a track, writing/erasing in units of sectors and the like, optically detectable guide tracks are formed concentrically or spirally on the optical disk, and a laser beam so focused as to have a diameter of 1 .mu.m or less is irradiated on a recording layer formed on a guide track to record information in the form of pits or a change in reflectance or transmittance.
In cases where digital information of variable data length is recorded, a track is divided into a plurality of sectors in order to increase recording efficiency so that recording/reproduction of information is carried out in units of 512 bytes or about 1K bytes. Each sector comprises an ID field containing track address and sector address information and a data field used for recording/reproduction of data.
Typically, a data format of data to be recorded on the data field has a sync-pattern signal block used to pull PLL (Phase Locked Loop) into synchronism, a data head identifying mark block standing for a sync signal which heads recorded data to identify the data head, a modulated data block and a resynchronization signal block for data word synchronization.
When recording data, an address information on an ID field of a target sector is read and data sets are recorded on a data field of the target sector at the timing of the address detection.
Incidentally, in cases where data is overwritten on a rewritable phase change optical disk, power of a semiconductor laser for generating a laser beam to be irradiated on the disk is typically controlled to be set to one of three levels of reproduction, erasing and recording modes. In the reproduction mode, a relatively low power beam which does not affect the phase structure of the recording film of the disk is irradiated on the disk and in the erasing and recording modes, a relatively high power beam sufficient to cause a phase change in crystalline/amorphous state is irradiated on the disk.
However, when data is overwritten reiteratively on one sector of the rewritable phase change optical disk, such a phenomenon occurs that the recording film is gradually deteriorated at the final portion of recorded data on a data field where the laser power for erasing/recording mode changes to that for reproduction mode, causing a defect which spreads forward from the final portion to give rise to an error in reproduction of the recorded data.
FIG. 7 illustrates at section (a) laser power levels at the final portion of the sector and at section (b) a recording format on the disk. As shown at (a) in FIG. 7, a beam of recording power or erasing power is irradiated, without exception, on the data recording block and a beam of reproduction power is irradiated, without exception, on the following gap and ID field.
Under this condition, a mechanism to be described below is considered to explain a cause of the aforementioned defect. Since the laser power for erasing/recording mode changes to that for reproduction mode at the same position on the final portion of the data recording block without exception, high thermal energy is generated owing to a phase change in the film occurring in the erasing/recording mode. This energy can not go over a thermal energy barrier at the following gap where only the low power beam for reproduction mode is irradiated, and the energy is accumulated in the final portion to deteriorate the recording film. In addition, the accumulated energy gradually migrates forward to spread the defect toward the trail of the recorded data. This phenomenon is a drawback inherent in so-called heat-mode recording.